Several of the most destructive diseases of important crop plants are caused by members of the taxonomic class Oomycetes, such as downy mildew (Plasmopara viticola) which affects grapes, and late blight (Phytophthora infestans) which affects potatoes. Control of most of these diseases is generally achieved by the repeated application of chemical fungicides. This form of control is often problematic. Chemical pesticides may be phytotoxic or may injure the plant to which they are applied under certain environmental conditions. Certain fungicides are perceived as undesirable by consumers and/or processors. For example, mancozeb and captan, two of the most effective chemical fungicides used against oomycetous plant pathogens, although allowed by the Environmental Protection Agency, are restricted or prohibited by major grape processors in the eastern United States. Furthermore, the development of resistance to certain chemical fungicides by oomycetous plant pathogens has limited the degree of disease control that can be achieved by their use. Metalaxyl, a newly developed fungicide, is no longer effective against grape downy mildew or potato late blight in many areas due to development of resistant strains of the pathogens.
There is therefore a need for alternative methods of controlling oomycetous plant pathogens. The present invention satisfies this need.